To be honest, I think you’re right in that he is comfortable with how things are and doesn’t want things to change. It really concerns me that it turns into a fight when you try and discuss the future - you’re absolutely within your rights to discuss what you want out of life! If you really want children, marriage, a house etc and he won’t even discuss those things, you would, sadly, be better off letting him go.
He either genuinely doesn’t know what he wants, and gets flustered when you bring the future up, or he’s cruising, stringing you along. Either way, it’s really not fair on you. I’m really sorry he’s being like this, though. It’s so hard when you love someone and they’re not willing to even meet you halfway.